1. Field of the Invention
The present invention relates to a sensor device having a binary output signal, and, more particularly, to a sensor device including a sensor element, especially a Hall sensor element, output signal preparation means, terminals for voltage supply, for a prepared output signal and for connection to ground and a circuit device for detecting faults in the sensor.
2. Prior Art
For determining the location of a component or an angular position, sensors are used whose output signal can assume essentially two states, namely one state in which the signal is "high" and one state in which it is "low".
Such sensors are Hall sensors or Hall switches, for instance, which scan movable parts on whose surface characteristic magnetic structures, for instance, are located. Depending on whether a north pole or a south pole is facing the Hall element, the output signal of the sensor is "high" or "low".
The design and mode of operation of integrated Hall effect sensors is described for instance in the journal Electronic Industry 7-1955. Printed in this journal is an article by P. Schieffer entitled "Integrierte Hall-Effekt-Sensoren zur Positions- und Drehzahlerkennung" [Integrated Hall Effect Sensors for Position and RPM Detection], which among other things describes how a Hall sensor switches over on the approach of a south pole of a magnet. The design of this Hall sensor is such that a chip that carries the Hall IC is connected to supply voltage by a first terminal and to ground via a resistor by a second terminal. The output signal of the Hall sensor can be picked up at this second terminal. A further input of the Hall IC is connected to supply voltage as well, via a further resistor. This input leads to a transistor, which is a component of the IC and acts as a so-called open-collector output stage.
On the aforementioned approach of a south pole of a magnet, the signal of the Hall sensor exceeds a turn-on threshold, and the open-collector output is made conducting; the signal present at the output of the Hall IC assumes a "low" state. If the magnet is moved away from the IC again, then the magnetic field corresponding to the output threshold is undershot, the open-collector output resumes high resistance, and the output signal of the Hall sensor changes to the "high" state.
With such Hall sensors, it is possible to detect not only north or south poles but also to detect a ferromagnetic baffle, for instance, that extends between the Hall IC and an opposed magnet. In general, such Hall effect sensors can be used in many possible applications. One example for the use of a plurality of Hall effect sensors is also disclosed in International Patent Application PCT/DE 95/00343. The embodiment described there shows how the steering angle of a motor vehicle is ascertained using a number of Hall sensors. This known system for steering angle detection comprises a precision system with a disk that has recesses within a track on its circumference. This rotatable disk is scanned with the aid of a plurality of Hall sensors, disposed at equal angular intervals, and by the cooperation of the disk and sensors a signal is produced that reproduces a code, which allows an absolute value detection within one steering wheel revolution. The disk is made for instance of sheet metal that is magnetically highly conductive, and it is permanently connected to the steering shaft. The recesses in the disk that rotates with the steering shaft are scanned with the aid of nine magnet-Hall-gates, disposed on the circumference of the disk. These Hall gates are a component of a Hall sensor at whose output a logical 1 ("high") is produced whenever there is no sheet metal between the magnet and the Hall gate, and a logical 0 ("low") is produced when a magnetic flux through a sheet-metal segment of the disk is interrupted.
Since in such a system, a univalent code is obtained for every angular position within 360.degree., a statement can be made about the angular position immediately after turn-on, as long as it is assured that all the Hall sensors are functioning properly. If a coarse system is used, with an encoded disk that rotates with a stepdown of 1:4 by means of a gear and is scanned with associated sensors, then the detectable angular range can be expanded to 7200.degree..
If the steering wheel angle sensor known from PCT/DE 95/00343 is to be used in conjunction with a driving dynamics control system, then intrinsic safety is required. This requirement means that all the Hall sensors of the system must either function reliably, or that any failure of a sensor can be detected as fast and reliably as possible.